macOS CI: use a cache of github.com/servo/servo

Relies on https://github.com/servo/taskcluster-config/pull/6

etc/taskcluster/decision_task.py

@@ -595,7 +595,7 @@ def update_wpt():
             "etc/taskcluster/macos/Brewfile-gstreamer",
         ])
         # Pushing the new changes to the git remote requires a full repo clone.
-        .with_repo(shallow=False)
+        .with_repo(shallow=False, alternate_object_dir="/var/cache/servo.git/objects")
         .with_curl_artifact_script(build_task, "target.tar.gz")
         .with_script("""
             export PKG_CONFIG_PATH="$(brew --prefix libffi)/lib/pkgconfig/"
@@ -631,19 +631,26 @@ def macos_wpt():
     def macos_run_task(name):
         task = macos_task(name).with_python2()
         return with_homebrew(task, ["etc/taskcluster/macos/Brewfile-gstreamer"])
-    wpt_chunks("macOS x64", macos_run_task, build_task, repo_dir="repo",
-               total_chunks=6, processes=4)
+    wpt_chunks(
+        "macOS x64",
+        macos_run_task,
+        build_task,
+        repo_dir="repo",
+        repo_kwargs=dict(alternate_object_dir="/var/cache/servo.git/objects"),
+        total_chunks=6,
+        processes=4,
+    )
 
 
 def wpt_chunks(platform, make_chunk_task, build_task, total_chunks, processes,
-               repo_dir, chunks="all"):
+               repo_dir, chunks="all", repo_kwargs={}):
     if chunks == "all":
         chunks = [n + 1 for n in range(total_chunks)]
     for this_chunk in chunks:
         task = (
             make_chunk_task("WPT chunk %s / %s" % (this_chunk, total_chunks))
             .with_treeherder(platform, "WPT-%s" % this_chunk)
-            .with_repo()
+            .with_repo(**repo_kwargs)
             .with_curl_artifact_script(build_task, "target.tar.gz")
             .with_script("tar -xzf target.tar.gz")
             .with_index_and_artifacts_expire_in(log_artifacts_expire_in)
@@ -887,7 +894,7 @@ def macos_build_task(name):
         # https://github.com/servo/servo/issues/24735
         .with_max_run_time_minutes(60 * 2)
         .with_env(**build_env, **unix_build_env, **macos_build_env)
-        .with_repo()
+        .with_repo(alternate_object_dir="/var/cache/servo.git/objects")
         .with_python2()
         .with_rustup()
         # Since macOS workers are long-lived and ~/.rustup kept across tasks:

etc/taskcluster/decisionlib.py

@@ -579,7 +579,7 @@ class UnixTaskMixin(Task):
         super().__init__(*args, **kwargs)
         self.curl_scripts_count = 0
 
-    def with_repo(self, shallow=True):
+    def with_repo(self, shallow=True, alternate_object_dir=None):
         """
         Make a shallow clone the git repository at the start of the task.
         This uses `CONFIG.git_url`, `CONFIG.git_ref`, and `CONFIG.git_sha`
@@ -592,14 +592,25 @@ class UnixTaskMixin(Task):
           `git` and `ca-certificate` need to be installed in the Docker image.
 
         """
+        # Not using $GIT_ALTERNATE_OBJECT_DIRECTORIES since it causes
+        # "object not found - no match for id" errors when Cargo fetches git dependencies
+        if alternate_object_dir:
+            self.with_env(ALTERNATE_OBJDIR=alternate_object_dir)
         return self \
         .with_env(**git_env()) \
         .with_early_script("""
             git init repo
             cd repo
+            {alternate}
             time git fetch --no-tags {depth} "$GIT_URL" "$GIT_REF"
             time git reset --hard "$GIT_SHA"
-        """.format(depth="--depth 30" if shallow else ""))
+        """.format(
+            depth="--depth 30" if shallow else "",
+            alternate=(
+                """echo "$ALTERNATE_OBJDIR" > .git/objects/info/alternates"""
+                if alternate_object_dir else ""
+            )
+        ))
 
     def with_curl_script(self, url, file_path):
         self.curl_scripts_count += 1